ATHENS, Greece -- The commander of U.S. naval forces in the Mediterranean Sea met Monday with Greece's chief Olympic security planner as authorities boosted scrutiny at ports and other points following the deadly terrorist attacks in Spain.

U.S. Vice Adm. Henry Ulrich, who directs the Navy's 6th Fleet, held talks with Public Order Minister Giorgos Voulgarakis in an unannounced visit just three days after Greece formally requested help from its NATO allies to safeguard the Aug. 13-29 Games.

No statements were made after the meeting. But the 6th Fleet, based in Gaeta, Italy, could likely play a central role in a NATO security network for the Olympics. Warships from the 6th fleet have been patrolling the eastern Mediterranean since the Sept. 11, 2001, attacks.

The meeting came just hours after a little known group claimed responsibility for a bomb planted outside an Athens branch office of a U.S. bank. Police used a controlled explosion late Sunday to destroy the small time bomb.

Greek authorities said security had been bolstered at airports, sea ports and train stations, but that overall planning for the games' security had not fundamentally changed.

Last Thursday's train bombings in Madrid heightened security worries in Athens - which is already spending a record $800 million on Olympic security.

"There is no country in the world that is safe today. No country in the world, no island, no place where one can claim to be secure and free of the danger of terrorism. And that's what's so terrifying in this war, which is a war with no face," Athens Mayor Dora Bakoyianni told Athens' Skai radio.

Ulrich's visit to Athens also coincided with a major two-week security exercise that began March 10 and involves about 400 U.S. troops and 1,500 Greek military and police personnel.

Security experts from Britain, Israel and France were scheduled to join the drills on Wednesday, state-run NET television said.

Athens' main concern remains the threat that Islamic militant groups, such as al-Qaida. But they also have to deal with attacks by Greek urban guerrilla groups and arson gangs.

On Monday, a group calling itself Revolutionary Struggle claimed responsibility for attempting to explode a bomb outside a Citibank branch in suburban Halandri.

The group, which gave no reason for the attack, had claimed responsibility for twin bombings Sept. 5 at Athens' main court complex. A police officer was injured in the blasts.

Anti-Olympics groups have in recent weeks carried out a number of firebomb attacks to protest the games.

A number of fringe groups, mostly self-proclaimed anarchists and arson gangs, have in the past targeted various businesses and diplomatic vehicles. Such groups have in recent years carried out hundreds of arson attacks and firebombings, mostly going after automatic teller machines at banks.